The present disclosure relates to a storage element and a storage device incorporating the storage element. The storage element includes a storage layer which stores a magnetization state of a ferromagnetic layer as information and a fixed magnetization layer in which a magnetization direction is fixed, and changes the magnetization direction of the storage layer by the flow of an electric current.
In information equipment such as a computer, as random access memory, DRAM (Dynamic Random Access Memory) which rapidly operates and is high-density has been widely used. However, since DRAM is a volatile memory which loses information when the power is turned off, a nonvolatile memory which does not lose information when the power is turned off has been anticipated.
As a candidate for nonvolatile memory, magnetic random access memory (MRAM) which stores information on the basis of the magnetization of a magnetic material has attracted attention and been studied. As a method of performing data storing in MRAM, for example, Japanese Unexamined Patent Application Publication No. 2004-193595 discloses a storage element using a spin torque magnetization reversal in which magnetization of a magnetic material which performs storage is reversed by a spin torque flowing between two magnetic materials. This element has attracted attention due to a relatively simple structure and a large number of rewritable times.
Similar to MRAM, a storage element using the spin torque magnetization reversal is configured a MTJ (Magnetic Tunnel Junction) in many cases. This configuration uses a phenomenon that, when spin polarized electrons which pass through a magnetic layer fixed to a given direction enter another free magnetic layer (where a direction is not fixed), a torque is applied to the magnetic layer (this phenomenon is also referred to as a spin transfer torque). In this case, when a current equal to or more than a given threshold is made to flow, the free magnetic layer is reversed. 0 and 1 are rewritten by changing a polarity of a current.
An absolute value of a current for the reversal is equal to or less than 1 mA for an element in the scale of about 0.1 μm. In addition, since this current value is reduced in proportion to the element volume, scaling is possible. Furthermore, since a word line which is necessary for generating the recording current magnetic field in MRAM is not necessary, there is also an advantageous effect in that a cell structure is simple.
Hereinafter, MRAM using the spin torque magnetization reversal is referred to as “a spin torque MRAM” or “ST-MRAM (Spin Torque-Magnetic Random Access Memory). The spin torque magnetization reversal may be sometimes referred to as a spin injection magnetization reversal.
As examples of ST-MRAM, there is ST-MRAM using an in-plane magnetization as disclosed in Japanese Unexamined Patent Application Publication No. 2004-193595 and ST-MRAM using a perpendicular magnetization as disclosed in Japanese Unexamined Patent Application Publication No. 2009-81215.
In ST-MRAM using the in-plane magnetization, a material can be selected with high degrees of freedom and a method of fixing the magnetization is relatively easy. However, in the case of the perpendicular magnetic layer, the material is limited to a material having a perpendicular magnetic anisotropy.
Recently, as disclosed in Nature Materials, Vol. 9, p. 721 (2010), for example, an interface anisotropy-type perpendicular magnetic layer using the perpendicular magnetic anisotropy which is developed at the crystal interface between Fe and an oxide has attracted attention.
When the interface anisotropy is used, the perpendicular magnetic layer can be obtained by a FeCoB alloy as the magnetic material and MgO as the oxide and thereby the trade-off between a high magnetoresistance ratio (MR ratio) and the perpendicular magnetization can be realized. Since the interface anisotropy acts favorable for both of the storage layer and a reference layer, the application to the perpendicular magnetization-type spin torque MRAM is expected.